Documentation: move rpc-cache.txt to filesystems/

This file is nfs-related.  (Maybe Documentation/filesystems/ would
benefit from a separate nfs/ directory at some point.)
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/00-INDEX

@@ -319,8 +319,6 @@ robust-futexes.txt
 	- a description of what robust futexes are.
 rocket.txt
 	- info on the Comtrol RocketPort multiport serial driver.
-rpc-cache.txt
-	- introduction to the caching mechanisms in the sunrpc layer.
 rt-mutex-design.txt
 	- description of the RealTime mutex implementation design.
 rt-mutex.txt

Documentation/filesystems/00-INDEX

@@ -84,6 +84,8 @@ relay.txt
 	- info on relay, for efficient streaming from kernel to user space.
 romfs.txt
 	- description of the ROMFS filesystem.
+rpc-cache.txt
+	- introduction to the caching mechanisms in the sunrpc layer.
 seq_file.txt
 	- how to use the seq_file API
 sharedsubtree.txt